The present invention relates to the protection of electrical and electronic circuits from high energy electrical overstress pulses or surges that might be injurious or destructive to the circuits, and render them non-functional, either permanently or temporarily. Suitable non-linear compositions and formulations of materials are known which can be connected to, or incorporated as part of an electrical circuit, and are characterized by high electrical resistance when exposed to low or normal operating voltages, but essentially instantaneously switch to low electrical resistance in response to an excessive or overstress voltage pulse, thereby shunting the excessive voltage or overstress pulse to ground. Structural arrangements are also known to facilitate the incorporation of such compositions into components of the circuit to be protected. The present invention relates particularly to a composite of composition and structure which combine to provide the overstress responsive environment.
The composite utilized in practicing the present invention is designed to respond substantially instantaneously to the leading edge of an overstress voltage pulse to change its electrical characteristics, and by shunting the pulse to ground, to thereby reduce the transmitted voltage of the pulse to a much lower value, and to clamp the voltage at that lower value for the duration of the pulse. This composite is also capable of substantially instantaneous recovery to its original high resistance value on termination of the overstress pulse, and of repeated responses to repetitive overstress pulses. For example, those composite can be designed to provide an ohmic resistance in the megohm range in the presence of low applied voltages in the range of 10 to more than 100 volts. However, upon the application of a sudden overstress pulse of, for example, 15,000 volts, this composite essentially instantaneously drops in resistance and switches to a low impedance shunt state that can reduce the overstress pulse to a value in the range of about a hundred volts, or less, and clamps the voltage at that low value for the duration of the pulse. In the present description, the high resistance state is called the "off-state", and the low resistance condition under overstress is called the "on-state".
Overstress responsive compositions and structures to facilitate the use of those compositions are known in the art. In general, those compositions constitute a mixture of finely divided electrically conductive and semiconductive particles supported in fixed spaced relation to each other in an electrically insulative resin binder or matrix. The most effective overstress composition is of the type described in U.S. Pat. No. 4,992,333 to Hyatt. In general, said composite comprises a distribution of different sized particles of conductive and semiconductive materials ranging from submicron to tens or a few hundreds of microns, with angstrom sized spacer particles, which are densely packed and bound with an insulative resin matrix. Other electrical overstress compositions are described and illustrated in other prior art patents.
U.S. Pat. No. 2,273,704 to R. O. Grisdale discloses a granular material having a non-linear voltage-current characteristic. This patent discloses a mixture of conductive and semiconductive granules that are coated with a thin insulative film (such as metal oxides), and are compressed and bonded together in a matrix to provide stable, intimate and permanent contact between the granules.
U.S. Pat. No. 4,097,834 to K. M. Mar et al. provides an electronic circuit protective device in the form of a thin film non-linear resistor, comprising conductive particles surrounded by a dielectric material, and coated onto a semiconductor substrate.
U.S. Pat. No. 2,796,505 to C. V. Bocciarelli discloses a non-linear precision voltage regulating element comprised of conductive particles having insulative oxide coatings thereon that are bound in a matrix. The particles are irregular in shape, and are point contiguous, i.e. the particles make point contact with each other.
U.S. Pat. No. 4,726,991 to Hyatt et al. discloses an electrical overstress protection material, comprised of a mixture of conductive and semiconductive particles, all of whose surfaces are coated with an insulative oxide film, and which are bound together in an insulative matrix, wherein the coated particles are in contact, preferably point contact, with each other.
U.S. patent application Ser. No. 07/666,026 to Stephenson and assigned to the assignee of the present invention, discloses a structural arrangement for the inclusion of these non-linear materials into electrical circuits.
Additional patents illustrative of the prior art in respect to this general type of non-linear resistor are U.S. Pat. Nos. 2,150,167 to Hutchins et al., 2,206,792 to Stalhana, 3,864,658 to Pitha et al., and 4,977,357 and 5,068,634 to Shrier.
As illustrated in the foregoing prior art, the overstress responsive compositions are usually positioned as a bulk material between a pair of contraposed electrodes, one of which is a signal electrode and the other is a ground electrode or a ground plane. The present invention, likewise functions to place a non-linear overstress responsive composite in a gap between a signal electrode and a ground electrode or ground plane. However, in the present invention, the composite is a hybrid of structural elements and composition. The key feature of the present invention resides in the use of a matrix or pattern of closely spaced electrically conductive metallic elements fixed on an electrically insulating substrate. These elements can conveniently be applied through screen printing, etching, vapor deposition, and the like, in a manner similar to the production of printed circuits. Preferably, the metallic elements are closely spaced and occur in a regular pattern, such as a uniform dot matrix, or a succession of closely spaced parallel lines or bars. For example, the spacing between the metallic elements may be from about 0.5 to about 200 microns depending on other factors to be described subsequently.
In addition to the pattern of closely spaced conductive elements, a coating composition is applied over the pattern covering and spanning the gap between the electrodes. The coating includes an insulative or dielectric resin whose dielectric strength is usually adjusted by the fill-ratio of insulative particles in a size range preferably of about 0.01 to about 50 microns. This coating may also include fine particulates of conductive and/or semiconductive materials. The resin (including any insulative fill particulate) may be present in an amount of from about 15 to 100% by volume of the coating. The conductive powder particles may range from 0 to about 60% by volume of the coating and have a particle size between about 0.01 and about 100 microns. The semiconductive powder particles may also range from 0 to about 60% by volume of the coating with particle sizes ranging between about 0.01 and about 50 microns. In such a composite of metallic elements and an overlying resin coating, with or without a particulate inclusion, the metallic elements function in a manner similar to conductive particles distributed in the resin matrix. However, in the present invention, these conductive metallic elements are arranged in a precisely controlled pattern, spatial distribution and spacing between conductive elements. The spacing of these elements can vary depending on the particulate inclusion in the resin, and the overstress electrical response properties desired for the composite.
The dielectric resin of the coating composition may preferably be a dielectric epoxy, silicone, or other resin, filled with insulative or pigment fillers, having suitable resistive or dielectric, mechanical and processing properties for the specific environment or application and production of the invention. It is preferred that the resin resistivity be from about 10.sup.12 to about 10.sup.15 ohms-meter. One suitable resin for the practice of the present invention is a silicone rubber marketed by General Electric Company as RTV-12, cured by heating.
If a conductive powder or particulate is to be included in the resin coating, it may be selected from a wide range of materials such as powders of carbon black, graphite, copper, aluminum, molybdenum, silver, gold, zinc, brass, cadmium, bronze, iron, tin, beryllium, lead, nickel, stainless steel, tantalum, titanium, tungsten and zirconium, the silicides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, iron and cobalt, and the borides, nitrides, aluminides and phosphides of the preceding elements which have resistivities of less than a few hundred ohm-cm. Preferably these conductive particles should be free of insulative or high resistance surface oxides, or the like.
If a semiconductor powder is to be included in the resin coating, it may be selected from a wide range of materials, such as silicon carbide, zinc oxide, bismuth oxide, other oxides such as oxides of calcium, niobium, vanadium, iron and titanium, the carbides of beryllium, boron and vanadium, the sulfides of lead, cadmium, zinc, and silver, indium antimonide, silicon, selenium, lead telluride, boron, tellurium, and germanium.
The invention is adaptable for application to various circuit components, interface connection devices, electronics packaging and may also be applied to a printed circuit. In the ensuing specific embodiments of the invention, it is illustrated as applied to a printed circuit and to a connector. While these applications are particularly useful modes for the invention, it is understood that they are presented simply for purposes of illustration, and the invention obviously is not limited thereto.
It is accordingly one object of the present invention to provide for the shunting to ground selectively high voltage overstress pulses or surges.
Another object of the present invention is to provide for the shunting to ground of high voltage overstress pulses or surges, by means of a composite interposed between a signal line and ground.
A further object of the present invention is to provide for such a composite which presents a very high resistance to ground in response to a normal or low voltage on the signal line, and a low resistance to ground in response to a high overstress voltage pulse on said signal line.
Still another object of the present invention is to provide for such a composite which includes an array of conductive elements located between an electrode associated with said signal line and an electrode connected to ground, and arranged in a predetermined and fixed pattern on an electrically insulating substrate, and further includes an electrically insulating resin coating overlying said elements and substrate.
And another object of the present invention is to provide such a composite wherein said electrically insulating resin coating includes conductive and/or semiconductive powder particles distributed therein.
Other objects and advantages of the present invention will become apparent to those skilled in the art from a consideration of the illustrative specific embodiments of the invention provided in the following description, had in conjunction with the accompanying drawings in which like reference characters refer to like or corresponding parts.